Prepreg Containing Epoxy Resin With Improved Flexural Properties

ABSTRACT

Prepreg that contains epoxy resin compositions that include an epoxy resin component and a curative powder comprising particles of 4,4′-diaminobenzanilide (DABA) wherein the size of the DABA particles is less than 100 microns and wherein the median particle size is below 20 microns.

This application is a divisional of co-pending U.S. application Ser. No.12/468,926 which was filed on May 20, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to high performance epoxy resinsthat are used in the aerospace industry. More particularly, the presentinvention relates to improving the flexural strength and strain tofailure of such epoxy resins.

2. Description of Related Art

Epoxy resins that are reinforced with a fibrous material, such as glassor carbon fiber, are used in a wide variety of situations where highstructural strength and low-weight are required. Composite materialsthat use a high performance epoxy resin matrix are especially popular inthe aerospace industry where weight and structural strength areimportant engineering and design considerations. High performance epoxyresins may include one or more thermoplastic materials that provide“toughening” of the epoxy resin. In addition, various combinations ofcurative agents are used to provide optimum curing and resin strength.Although such high performance epoxy resin composite materials aredesirable because of their relatively high strength to weight ratio,they do present some specific issues with respect to flexibility andflexural properties. The flexural properties of an epoxy resin areimportant for design considerations because the overall strength, damagetolerance and resistance to impact of composite parts made using suchepoxy resins are dependent upon these properties.

Flexural strength, flexural modulus and strain to failure are flexuralproperties of a cured epoxy resin that are routinely measured in theaerospace industry. The flexural strength of a cured epoxy resin isdefined as its ability to resist deformation under a load. The flexuralstrength is determined by measuring the amount of force or load that isrequired to make a cured epoxy resin test specimen fail. For materialsthat deform significantly without breaking, the load at failure is thepoint at which the specimen's resistance to bending drops dramatically.The flexural modulus is the ratio of the stress (load) to strain(flexing) during deformation of the cured epoxy resin. The flexuralmodulus is determined by using the values obtained during testing offlexural strength to calculate the flexural modulus. Strain to failureis a measure of the degree to which a specimen will bend (strain) beforeit fails.

ASTM D790 and ISO 178 are two standard test procedures that are used todetermine the flexural properties of cured epoxy resins. These twoprocedures are basically the same. A test specimen is supported on asupport span and the load (stress) is applied to the center by a loadingnose to produce a three-point bending (strain) at a specified rate. Thevarious parameters for the test procedure include the size of thesupport span, the speed of loading and the maximum deflection for thetest. These parameters depend upon the size of the test specimen, whichdiffers between the ASTM D970 and ISO 178 protocols. A common size forthe test specimen is 3.2 mm×12.7 mm×125 mm for the ASTM D790 test and 10mm×4 mm×80 mm for the ISO 178 tests.

The development of high performance epoxy resins where the flexuralstrength and strain to failure are made as high as possible withoutdeleteriously affecting the flexural modulus has been, and continues tobe, a major goal in the aerospace composites industry.

Epoxy resin formulations typically include one or more curative agents.One such curative agent is 4,4′-diaminobenzanilide (DABA). DABA istypically supplied as a powder that is mixed directly with the epoxyresin. It would be desirable to provide epoxy resins that are cured withDABA and which exhibit improved flexural strength and strain to failurewithout negatively affecting the flexural modulus of the DABA-curedepoxy resin.

SUMMARY OF THE INVENTION

In accordance with the present invention, it was unexpectedly discoveredthat the flexural strength and strain to failure of high performanceepoxy resins can be increased if a powder containing specially sizedparticles of 4,4′-diaminobenzanilide (DABA) is used as the curativeagent. It was also discovered that these unexpected increases inflexural strength and strain to failure could be achieved while keepingthe flexural modulus at or above levels that are comparable to existinghigh performance epoxy resins that are cured using conventionalcuratives. In addition, the cured resins of the present inventionexhibit relative high glass transition temperatures (T_(g)).

The present invention covers uncured epoxy resin compositions thatinclude an epoxy resin component and a curative powder that is made upof 4,4′-diaminobenzanilide (DABA) particles which have sizes that areless than 100 microns and wherein the median particles size is below 20microns. Epoxy resins that are cured with this specially sized DABAcurative powder have flexural strengths and strain to failure levelsthat are significantly higher than those observed for epoxy resins curedwith commercially available DABA powders that contain larger particlesizes. The cured resins were also found to have an unexpectedly highglass transition temperature.

In addition to the uncured epoxy resin composition, the presentinvention covers use of the epoxy resin composition as the matrix resinfor prepreg as well as other combinations of the uncured epoxycomposition with fibrous materials. In addition, the invention coverscured epoxy resin compositions and fiber reinforced composite partswherein the resin matrix is a cured epoxy resin composition inaccordance with the present invention. The invention also covers methodsfor making uncured epoxy resin compositions and methods for making curedparts that incorporate the epoxy resin composition.

The above described and many other features and attendant advantages ofthe present invention will become better understood by reference to thefollowing detailed description when considered in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the particle size distribution of DABAcurative powder as received from a commercial supplier.

FIG. 2 is a graph showing the particle size distribution of DABAcurative powder in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Epoxy resin compositions in accordance with the present invention may beused in a wide variety of situations where a cured epoxy resin having ahigh flexural strength and strain to failure is desired. The epoxy resincompositions are also useful in those situations where glass transitiontemperatures (T_(g)) of the cured resin above 220° C. are desired.Although the epoxy resin compositions may be used alone, thecompositions are generally combined with a fibrous support to formcomposite materials. The composite materials may be in the form of aprepreg or cured final part. Although the composite materials may beused for any intended purpose, they are preferably used in aerospaceapplications for both structural and non-structural parts.

For example, the epoxy resin may be used to form composite material thatis used in structural parts of the aircraft, such as fuselages, wingsand tail assemblies. The epoxy resin may also be used to make compositematerial parts that are used in non-structural areas of the airplane.Exemplary non-structural exterior parts include engine nacelles andaircraft skins. Exemplary interior parts include the aircraft galley andlavatory structures, as well as window frames, floor panels, overheadstorage bins, wall partitions, wardrobes, ducts, ceiling panels andinterior sidewalls.

The epoxy resin compositions of the present invention include from 55 to75 weight percent of an epoxy resin component that includes one or moreepoxy resins. The epoxy resins may be selected from any of the epoxyresins that are used in high performance aerospace epoxies.Difunctional, trifunctional and tetrafunctional epoxy resins may beused. Preferably, the epoxy resin component will be made upsubstantially of a trifunctional epoxy compound. If desired,tetrafunctional epoxies may be included. The relative amounts oftrifunctional and tetrafunctional epoxies may be varied. However, it ispreferred that the amount of trifunctional epoxy is greater than orequal to the amount of tetrafunctional epoxy.

A trifunctional epoxy resin will be understood as having the three epoxygroups substituted either directly or indirectly in a para or metaorientation on the phenyl ring in the backbone of the compound. Atetrafunctional epoxy resin will be understood as having the four epoxygroups in the backbone of the compound. Suitable substituent groups, byway of example, include hydrogen, hydroxyl, alkyl, alkenyl, alkynyl,alkoxyl, aryl, aryloxyl, aralkyloxyl, aralkyl, halo, nitro, or cyanoradicals. Suitable non-epoxy substituent groups may be bonded to thephenyl ring at the para or ortho positions, or bonded at a meta positionnot occupied by an epoxy group.

Suitable trifunctional epoxy resins, by way of example, include thosebased upon: phenol and cresol epoxy novolacs; glycidyl ethers ofphenol-aldelyde adducts; aromatic epoxy resins; dialiphatic triglycidylethers; aliphatic polyglycidyl ethers; epoxidised olefins; brominatedresins, aromatic glycidyl amines and glycidyl ethers; heterocyclicglycidyl imidines and amides; glycidyl ethers; fluorinated epoxy resinsor any combination thereof. A preferred trifunctional epoxy is thetriglycidyl ether of para aminophenol, which is available commerciallyas Araldite MY 0500 or MY 0510 from Huntsman Advanced Materials(Monthey, Switzerland). Another preferred trifunctional epoxy resin istriglycidyl meta-aminophenol. A particularly preferred trifunctionalepoxy is triglycidyl meta-aminophenol, which is available commerciallyfrom Huntsman Advanced Materials (Monthey, Switzerland) under the tradename Araldite MY0600, and from Sumitomo Chemical Co. (Osaka, Japan)under the trade name ELM-120.

Suitable tetrafunctional epoxy resins, by way of example, include thosebased upon: phenol and cresol epoxy novolacs; glycidyl ethers ofphenol-aldelyde adducts; aromatic epoxy resins; dialiphatic triglycidylethers; aliphatic polyglycidyl ethers; epoxidised olefins; brominatedresins, aromatic glycidyl amines and glycidyl ethers; heterocyclicglycidyl imidines and amides; glycidyl ethers; fluorinated epoxy resinsor any combination thereof. A preferred tetrafunctional epoxy isN,N,N′,N′-tetraglycidyl-m-xylenediamine, which is available commerciallyas Araldite MY0720 or MY0721 from Huntsman Advance Materials (Monthey,Switzerland).

If desired, the epoxy resin component may also include a difunctionalepoxy, such a Bisphenol-A (Bis-A) or Bisphenol-F (Bis-F) epoxy resin.Exemplary Bis-A epoxy resin is available commercially as Araldite GY6010(Huntsman Advanced Materials) or DER 331, which is available from DowChemical Company (Midland, Mich.). Exemplary Bis-F epoxy resin isavailable commercially as Araldite GY281 and GY285 (Huntsman AdvancedMaterials). The amount of Bis-A or Bis-F epoxy resin present in theepoxy resin component may be varied. It is preferred that no more than20 weight percent of the total epoxy resin component be difunctionalepoxy resin.

The epoxy resin component may optionally include from 5 to 15 weightpercent of a thermoplastic toughening agent. Thermoplastic tougheningagents are well-know for use in preparing high performance epoxy resins.Exemplary toughening agents include polyether sulfone (PES),polyetherimide (PEI), polyamide (PA) and polyamideimide (PAI). PES isavailable commercially from a variety of chemical manufacturers. As anexample, PES is available from Sumitomo Chemical Co. Ltd. (Osaka, Japan)under the tradename Sumikaexcel 5003p. Polyetherimide is availablecommercially as ULTEM 1000P from Sabic (Dubai). Polyamideimide isavailable commercially as TORLON 4000TF from Solvay Advanced Polymers(Alpharetta, Ga.). The thermoplastic component is preferably supplied asa powder that is mixed in with the epoxy resin component prior toaddition of the curative agent.

The epoxy resin composition may also include additional ingredients,such as performance enhancing and/or modifying agents provided that theyalso do not adversely affect the flexural strength, strain to failureand flexural modulus of the cured resin. The performance enhancing ormodifying agents, for example, may be selected from: flexibilizers,particulate fillers, nanoparticles, core/shell rubber particles, flameretardants, wetting agents, pigments/dyes, conducting particles, andviscosity modifiers. It is preferred that the resin composition does notinclude additional ingredients. It is preferred that the resincomposition be limited to the epoxy component and the specially sizedcurative powder, as described below. More preferably, the resincomposition will be composed of a trifunctional epoxy and the speciallysized curative powder as the curative agent. Most preferred is thecombination of specially sized DABA curative powder and ameta-substituted trifunctional epoxy, such as MY0600.

In accordance with the present invention, the epoxy resin component iscured using powdered 4,4′-diaminobenzanilide (DABA) as the curativeagent. The powdered DABA curative is preferably prepared by takingcommercially available DABA powder and passing it through a No. 400sieve (0.0015 inch openings). The particle size distribution of atypical commercially available DABA powder is shown in FIG. 1. Such DABApowder is available from any number of commercial sources. Exemplarycommercial suppliers include Acros Organics (Fair Lawn, N.J.) and AlfaAesar (Ward Hill, Mass.). The powders are generally at least 95 weight %pure DABA and more typically are at least 98 weight % pure DABA. Theparticle size distribution of the commercial powder was determined usinga Horiba LA-500 Particle Size Analyzer. The “as received’ commercialpowder included particles as large as 200 microns and as small as 0.2microns. The median particle size is about 48 microns and the meanparticle size is about 53 microns. About 38 percent of the particleshave a particle size of between 10 and 50 microns and about 14 percentof the particles have particle sizes that are over 100 microns. About 40percent of the particles have particle sizes between 50 and 100 microns.

When the above described commercial DABA powder is passed through a No.400 sieve, the resulting specially sized powder has a particledistribution curve as shown in FIG. 2, as measured using the HoribaLA-500 Particle Size Analyzer. Specially sized powdered DABA curativeshaving the particle size distribution as shown in FIG. 2 provideflexural strengths, strain to failure and T_(g) levels that aresignificantly higher than those observed for epoxy resins cured withcommercially available DABA powders that contain larger particles asshown in FIG. 1.

The specially sized DABA powder in accordance with the present inventionshould have few, if any, particles that are larger than 100 microns. Thespecially sized powder will also contain few, if any, particles that aresized below about 0.1 micron. Smaller particle sizes are possible andmay be included in the specially sized powder. However, conventionalgrinding and sieving techniques generally do not produce a large numberof particles that are smaller than 0.1 micron. Accordingly, thepreferred lower limit for particle size is about 0.1 micron. The medianparticle size for the powder should be below 20 microns. Preferably, themedian particle size will be between 10 and 20 microns with theparticularly preferred median particle size being about 15 microns. Themean particle size for the powder should also be below 20 microns.Preferably, the mean particle size will be between 10 and 20 micronswith the particularly preferred mean particle size being about 17microns. At least 70 percent of the particles should have particle sizesof below 50 microns. Preferably about 85 percent of the particles willhave particle sizes of below 50 microns and most preferred are powderswere at least 95 percent of the particles have particle sizes below 50microns. It is also preferred that at least about 16 percent of theparticles have particle sizes of less than 5 microns.

The specially sized DABA powder may be made in any number of waysprovided that the above described particle size distribution isobtained. For example, relatively large pieces of DABA may be ground upand passed through various sieves to obtain a particles sizedistribution that is similar to the commercially available powder shownin FIG. 1. The resulting powder is then passed through a No. 400 sieveto obtain a powder made up of particles that meet the particle sizedistribution ranges set forth above. It is preferred that a powderhaving a particle size distribution that is the same or similar to thatshown in FIG. 1 is purchased or prepared and then passed through a No.400 sieve to obtain the specially sized DABA powder. If desired, thecommercially obtained powder having a particle size distribution asshown in FIG. 1 may be ground further prior to passing through a No. 400sieve. The specially sized DABA powder should be at least 95 weight %DABA. More preferably, the powder should be at least 98 weight % pureDABA.

The amount of specially sized DABA curative powder that is mixed withthe epoxy resin component to form the uncured epoxy resin compositionmay be varied so as to provide cured resins having flexural strengths ofat least 25 ksi and strain to failure values of at least 4 percent, asmeasured using ASTM D970. The flexural modulus should be about 790 ksior higher and the T_(g) should be 220° C. or higher. Preferably, thestoichiometric ratio of epoxy component to DABA will be between 1.0 to1.0 and 1.0 to 0.6. The preferred stoichiometric ratio of epoxycomponent to DABA is about 1.0 to 0.85. Minor amounts of other curativesmay be included in the epoxy resin composition. However, it is preferredthat at least 80 weight percent of the curative for the epoxy resincomponent be the specially sized DABA curative powder. Most preferredare uncured epoxy resin composition where the curative is at least 95weight percent of the specially sized DABA curative powder. Exemplaryother curatives that may be added include 3,3′-diaminodiphenylsulfone(3,3′-DDS) and 4,4′-diaminodiphenylsulfone (4,4′-DDS).

The epoxy resin composition of the present invention is made inaccordance with standard resin processing procedures for highperformance epoxy resins. If more than one epoxy is used, the epoxyresins are mixed together at room temperature to form an epoxy resincomponent. Any thermoplastic component or other additive is added andthe mixture is then heated, if necessary, to dissolve the thermoplasticor other additive. The mixture is then cooled down, if necessary, to atemperature that is 65° C. or below (preferably room temperature) andthe specially sized DABA curative powder is mixed into the resin mixtureto form the final uncured epoxy resin composition. The DABA curativepowder should not be dissolved prior to addition to the epoxy resincomponent. As shown in Comparative Example 3, pre-dissolving the DABApowder in solvent, as an alternative to reducing the particle size ofthe powder, does not provide the improved flexural properties obtainedwhen the specially sized DABA powder is used as the curative agent.

The uncured epoxy resin composition may be used in any application wherea high performance epoxy resin is needed. However, the principal use forsuch resins is in combination with a fibrous reinforcement to form aprepreg that is later used to form a cured composite part. The uncuredepoxy resin composition is applied to the fibrous reinforcement inaccordance with any of the known prepreg manufacturing techniques. Thefibrous reinforcement may be fully or partially impregnated with theepoxy resin composition during formation of the prepreg. The prepreg istypically covered on both sides with a protective film and rolled up forstorage and shipment at temperatures that are typically kept well belowroom temperature to avoid premature curing. Any of the other prepregmanufacturing processes and storage/shipping systems may be used, ifdesired.

The fibrous reinforcement may be selected from hybrid or mixed fibersystems that comprise synthetic or natural fibers, or a combinationthereof. Exemplary preferred fibrous reinforcement materials includefiberglass, carbon fibers or aramid (aromatic polyamide) fibers. Thefibrous reinforcement is preferably composed of carbon fibers.

The fibrous reinforcement may comprise cracked (i.e. stretch-broken) orselectively discontinuous fibers, or continuous fibers. The fibrousreinforcement may be in a woven, non-crimped, non-woven, unidirectional,or multi-axial textile structure form, such as quasi-isotropic choppedpieces of unidirectional fibers. The woven form may be selected from aplain, satin, or twill weave style. The non-crimped and multi-axialforms may have a number of plies and fiber orientations. Such styles andforms are well known in the composite reinforcement field, and arecommercially available from a number of companies, including HexcelReinforcements (Villeurbanne, France).

The prepreg may be in the form of continuous tapes, towpregs, webs, orchopped lengths (chopping and slitting operations may be carried out atany point after the epoxy resin composition is impregnated into thefibrous reinforcement). The prepreg may be used as an adhesive orsurfacing film and may additionally have embedded carriers in variousforms both woven, knitted, and non-woven.

The prepreg may be molded using any of the standard techniques used toform composite parts. Typically, one or more layers of prepreg are placein a suitable mold and cured to form the final composite part. Prepregcontaining the uncured epoxy resin composition of the invention may befully or partially cured using any suitable temperature, pressure, andtime conditions known in the art. Typically, the prepreg will be curedin an autoclave at temperatures of between 160° C. and 190° C. withcuring temperatures of between about 175° C. and 185° C. beingpreferred. Compression molding of quasi-isotropic chopped prepreg ormolding material is a preferred procedure. The quasi-isotropic choppedprepreg is the same as HexMC® compression molding material that isavailable from Hexcel Corporation (Dublin, Calif.), except that theresin component of this quasi-isotropic chopped prepreg is made inaccordance with the present invention. Such quasi-isotropic materialsare described in EP 113431 B1 and U.S. patent application Ser. No.11/476,965.

Examples of practice are as follows:

Comparative Example 1

A comparative epoxy resin sample was prepared in which 20.00 g of MY600(triglycidyl meta-aminophenol) epoxy was mixed with 9.14 g of 3,3′-DDSand 1.02 g of 4,4′-DDS, which are conventional curing agents that areused to cure high performance epoxy resins. The epoxy and curativeagents were mixed together at room temperature and the resulting resinwas cured at 177° C. for 2 hours to form cured resin samples that weretested according to ASTM D790. The flexural modulus of the cured resinsample was 749 ksi. The flexural strength was 31.1 ksi and the strain tofailure was 4.0%. The T_(g) of the cured resin was 205° C.

Comparative Example 2

A comparative epoxy resin sample was prepared in which 21.20 g of MY600epoxy was mixed with 9.86 g of DABA curative powder (as received fromthe supplier), which provided a stoichiometric ratio of MY600 to DABA of1:0.85. The particle size distribution of the DABA curative powder isshown in FIG. 1. The median particle size for the powder was about 48microns and the mean particle size was about 53 microns. About 14percent of the particles had particle sizes of between 100 and 200microns and about 8 percent had particle sizes of less than 15 microns.About 70% of the particles had particle sizes between 150 microns and 30microns

The epoxy and DABA curative powder were mixed together at roomtemperature and the resulting resin was cured at 177° C. for 2 hours toform cured resin samples that were tested according to ASTM D790. Theflexural modulus of the cured resin sample was 745 ksi. The flexuralstrength was 12.2 ksi and the strain to failure was 2.4%. The T_(g) ofthe cured resin sample was 214° C.

Comparative Example 3

Three comparative epoxy resin samples (CA, CB and CC) were prepared bymixing MY600 epoxy with differing amounts of DABA curative. Instead ofadding the DABA curative powder directly to the epoxy resin as inComparative Example 2, the powders were dissolved in dioxolane prior tobeing added to the resin. Resin CA contained 17.92 g of MY600 and 10.11g of DABA, which provided a stoichiometric ratio of MY600 to DABA of1:1. Resin CB contained 21.20 g of MY600 and 9.86 g of DABA(stoichiometric ratio of 1:0.85). Resin CC contained 21.75 g of MY600and 8.82 g of DABA (stoichiometric ratio of 1:0.75).

The epoxy and dissolved DABA powders were mixed together at roomtemperature to form the three comparative mixtures. The mixtures wereheated from room temperature to 50° C. under 30 inches Hg for about 2hours to evaporate the solvent. The resulting resins (CA, CB and CC)were cured at 177° C. for 2 hours to form cured comparative resinsamples that were tested according to ASTM D790. The flexural modulii ofthe cured resin samples were: 825 ksi for Resin CA; 863 ksi for ResinCB; and 816 ksi for Resin CC. The flexural strengths were: 13.9 ksi forResin CA; 17.4 ksi for Resin CB; and 19.2 ksi for Resin CC. The strainsto failure were: 1.7% for Resin CA; 2.0% for Resin CB; and 2.4% forResin CC. The T_(g)'s of the cured resins were 202° C., 219° C. and 199°C. for Resins CA, CB and CC, respectively.

Example 1

Three exemplary epoxy resin samples (A, B and C) were prepared by mixingMY600 epoxy with differing amounts of DABA curative powder. Instead ofadding the DABA curative powder directly to the epoxy resin as inComparative Example 2, the powder was first passed through a 400 meshscreen to provide a powder having the particles size distribution asshown in FIG. 2. The median particle size for the powder was about 15microns and the mean particle size was about 16 microns. None of theparticle sizes were above about 100 microns and none of the particleshad sizes less than 0.1 micron. About 3 percent of the particles hadparticle sizes of between 50 and 100 microns and about 25 percent of theparticles had sizes of less than 10 microns. About 75% of the particleshad particle sizes between 50 microns and 10 microns

Resin A contained 17.92 g of MY600 and 10.11 g of reduced-size DABA,which provided a stoichiometric ratio of MY600 to DABA of 1:1. Resin Bcontained 21.20 g of MY600 and 9.86 g of reduced-size DABA(stoichiometric ratio of 1:0.85). Resin C contained 21.75 g of MY600 and8.82 g of reduced-size DABA (stoichiometric ratio of 1:0.75).

The epoxy and reduced-size DABA powders were mixed together at roomtemperature to form the three exemplary resin samples A, B and C. Theresulting resins were cured at 177° C. for 2 hours to form cured resinsamples that were tested according to ASTM D790. The flexural modulus ofthe cured resin samples were: 794 ksi for Resin A; 794 ksi for Resin B;and 801 ksi for Resin C. The flexural strengths were: 27.7 ksi for ResinA; 33.6 ksi for Resin B; and 30.2 ksi for Resin C. The strains tofailure were: 4.6% for Resin A; 5.6% for Resin B; and 4.9% for Resin C.The T_(g)'s of the cured resins were 229° C., 234° C. and 227° C. forResins A, B and C, respectively.

As can be seen from the above examples, the flexural strength and strainto failure is significantly and unexpectedly higher for Example 1 thanfor Comparative Examples 2 and 3. In accordance with applicant'sinvention, it was discovered that epoxy resins cured with thereduced-size DABA curative powder of Example 1 achieved flexuralstrength and strain to failure levels that could not be achieved withlarger sized DABA particles (Comparative Example 2) or smaller sizedparticles (i.e. dissolved DABA powder) as shown in Comparative Example3. The flexural modulus of the resins cured with reduced-size DABA inaccordance with the present invention also remain at a relatively highlevel. In addition, the T_(g)'s of the exemplary resins wereunexpectedly higher that the T_(g)'s of the comparative resins.

The resins in accordance with the present invention (A, B and C) haveflexural strengths and strain to failure levels that are in the samerange as epoxy resins cured with conventional curatives (3,3′-DDS and4,4′-DDS) as shown in Comparative Example 1. In addition, the flexuralmodulus of the DABA-cured resins of the present invention is as high orhigher than the flexural modulus of comparable epoxy resins cured withconventional curatives as set forth in Comparative Example 1.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited by the above-describedembodiments, but is only limited by the following claims.

1. A prepreg comprising: a fibrous reinforcement; and an uncured epoxyresin composition comprising: an epoxy resin component; and a curativepowder comprising particles of 4,4′-diaminobenzanilide wherein the sizeof said particles is less than 100 microns and wherein the medianparticle size is below 20 microns.
 2. A prepreg according to claim 1wherein said the median particle size is between 10 microns and 20microns.
 3. A prepreg according to claim 1 wherein said epoxy resincomponent consists essentially of a trifunctional epoxy.
 4. A prepregaccording to claim 1 wherein the stoichiometric ratio of said epoxycomponent to said 4,4′-diamino benzanilide is between 1.0 to 1.0 and 1.0to 0.7.
 5. A prepreg according to claim 4 wherein the stoichiometricratio of said epoxy component and said 4,4′-diamino benzanilide is about1.0 to 0.85.
 6. A prepreg according to claim 1 wherein at least 70percent of the particles in said curative powder have a particle size ofbelow 50 microns.
 7. A cured laminate comprising prepreg according toclaim 1 that has been cured.
 8. A cured laminate comprising prepregaccording to claim 2 that has been cured.
 9. A cured laminate comprisingprepreg according to claim 3 that has been cured.
 10. A cured laminatecomprising prepreg according to claim 4 that has been cured.
 11. A curedlaminate according to claim 7 which has a flexural strength of at least25 ksi and a strain to failure of at least 4.0 percent.
 12. A method formaking a prepreg comprising the step of combining a fibrousreinforcement with an uncured epoxy resin composition comprising: anepoxy resin component; and a curative powder comprising particles of4,4′-diaminobenzanilide wherein the size of said particles is less than100 microns and wherein the median particle size is below 20 microns.13. A method for making a prepreg according to claim 12 wherein saidmedian particle size is between 10 microns and 20 microns.
 14. A methodfor making a prepreg according to claim 12 wherein said epoxy resincomponent consists essentially of a trifunctional epoxy.
 15. A methodfor making a prepreg according to claim 12 wherein the stoichiometricratio of said epoxy component to said 4,4′-diamino benzanilide isbetween 1.0 to 1.0 and 1.0 to 0.7.
 16. A method for making a prepregaccording to claim 12 wherein at least 70 percent of the particles insaid curative powder have a particle size of below 50 microns.
 17. Amethod for making a cured laminate comprising the step of curing prepregaccording to claim
 1. 18. A method for making a cured laminatecomprising the step of curing prepreg according to claim
 2. 19. A methodfor making a cured laminate comprising the step of curing prepregaccording to claim
 3. 20. A method for making a cured laminatecomprising the step of curing prepreg according to claim 4.